Photochromic glass

ABSTRACT

This invention relates to the production of photochromic glasses having compositions within the CdO B2O3 SiO2 field.

United States Patent Inventor Appl. No.

Filed Patented Assignee Gerald S. Meiling Medford, Mass. 761,802

Sept. 23, 1968 Oct. 26, 1971 Corning Glass Works New York, N.Y.

PHOTOCHROMIC GLASS 3 Claims, No Drawings US. Cl .L.

106/54, lO6/DIG. 6

Int. Cl C03c 3/04 Field of Search 106/52, 54

Primary Examiner-Helen M. McCarthy Attorneys-Clarence R. Patty, Jr. andClinton S. lanes, Jr.

ABSTRACT: This invention relates to the production of photochromicglasses having compositions within the CdO B 0 SiO field.

PHOTOCHROMIC GLASS The development of practical photochromic orphototrophic glasses, as such have been variously termed, originated inU.S. Pat. No. 3,208,860. This patent discloses the production ofphotochromic glass articles, i.e., glass articles which will darken uponthe impingement of actinic radiation thereon, e.g., sunlight, but whichwill return to their original state when the actinic radiation isremoved. Such glasses can impart a new dimension to the ophthalmicindustry since sunglasses may be produced that will protect the wearerseyes from bright sunlight but which will become clear when the wearermoves out of the sunlight. Likewise, in the building industry, windowsproduced from such glasses will reduce glare from sunlight and, also,substantially decrease the amount of heat transmitted into a room fromthe sunlight. And, inasmuch as the amount of darkening exhibited by theglass would be proportional to the intensity of the light impingingthereupon, such a glass would be far superior to the presently availabletinted windows which have an invariant optical transmittance value.

U.S. Pat. No. 3,208,806 discloses inorganic silicate glasses containingvery fine-grained crystals of silver chloride and/or silver bromideand/or silver iodide which exhibit phototropic behavior, i.e., theseglasses have optical transmittances that vary reversibly with theintensity of actinic radiation incident thereon. Thus, as is explainedin the patent, the silver halide crystals become darker in color uponexposure to actinic radiation (commonly radiation of wavelengths betweenabout 0.3-0.5 microns), but return to their original color when theactinic radiation is removed. As is stated in the patent, the mechanismfor this behavior is not fully understood but is believed to involve areaction between the actinic radiation and the silver halide crystalsdispersed in the glassy matrix, this reaction altering the absorptivequalities of the crystals to visible radiations. Nevertheless, inasmuchas the crystals are dispersed and encased within an amorphous or glassymatrix, the removal of the actinic radiation allows the crystals toreturn to their former state since the glassy matrix is impermeable toand nonreactive with the reaction products formed during such exposureso that they, therefore, cannot diffuse away.

U.S. Pat. No. 3,325,299 discloses inorganic silicate glasses containingvery fine-grained crystals of cadmium and/or copper chloride and/orbromide and/or iodide which also exhibit phototropic behavior. Here, asin the case of the silver halide crystals of U.S. Pat. No. 3,208,860,the cadmium and/or copper halide crystals darken under the influence ofactinic radiation of the same wavelengths and will return to theiroriginal state upon withdrawal of the radiation. The mechanism for thephototropic behavior, while not understood, is believed to be similar tothat obtaining for the silver halides.

Both of these patents describe the manufacture of practical photochromicglasses. Hence, whereas phototropic articles such as sunglasses madefrom organic plastic materials were commercially available, thosearticles quickly demonstrated fatigue, i.e., after several successiveexposures to and removals from actinic radiation, these articles beganto lose their phototropic behavior. The phototropic properties of theglasses disclosed in the above-recited patents, however, remainedessentially constant after innumerable cycles of exposures and removals.

l have discovered that a small group of glasses within the CdO-B,O-composition field, essentially free from crystals of any kind, can beproduced which exhibit good photochromic properties and which areessentially free from fatigue. Thus, glass articles consistingessentially, in mole percent on the oxide basis, of about 45-70 percentCdO, -45 percent 3,0,, and 5-35 percent SiO will darken when subjectedto actinic radiation of wavelengths between about 0.3-0.5 microns(3,000A.-5,000A.) but will return to their original optical density whenthis radiation is withdrawn.

Table l reports several glass compositions, in mole percent and weightpercent on the oxide basis, which are demonstrative of my invention. Thebatch components may consist of any materials, e.g., the metallicelements, oxides, or other compounds which, on being melted together,are converted to the desired oxide compositions in the properproportions, ln producing each glass recorded in table I, the batchmaterials were thoroughly mixed together and melted in an air atmospherein covered platinum crucibles at l,200 C. for 4 hours, the melts beingstirred with a platinum stirrer to achieve good homogenization. Themelts were poured into stainless steel molds preheated to 300 C. and themolds containing the melts transferred to an annealer operating at 500C. Samples (50 mm.X7 mm.X2 mm.) were cut from each of the molded glassshapes and fire polished for the optical mea surements.

The quantities of CdO, 8,0 and SiO, must be held within theabove-recited ranges therefor in order to obtain transparent glassesexhibiting good photochromic behavior. Hence, where the amount of CdO isgreater than about mole percent, the melt devitrifies as it is beingcooled to a glass and where the amount of CdO is less than about 45 molepercent, two immiscible liquids result leading to an opal glass.However, although the amounts of CdO, B 0 and SiO, are critical to theinvention, minor amounts of compatible metal oxides such as M20, K 0,CaO, SrO, ZnO, PhD, and MgO may be included to modify the melting andforming qualities of the glass. Fluorine may also be added to the batchto improve melting and reduce the tendency of the melt to devitrifyduring cooling. The total of all such additions ought not to exceedabout 10 mole percent.

Doping of the glasses with copper and/or silver (up to about 0.05 weightpercent CuO and up to about 0.5 weight percent Ag) appears to improvethe sensitivity thereof to the actinic radiation and to increase therate of darkening and fading exhibited by the glasses upon exposure toand removal from the actinic radiation.

An X-ray diffraction analysis and electron microscope examination ofthese glasses demonstrated the absence of bona fide crystals. An EPRspectrum (electron paramagnetic resonance) of the cadmium borosilicateglass was obtained in an attempt to discover the mechanism for thephotochromic behavior. Examination of this spectrum suggested thepossibility that Cd is responsible for the photochromic behavior butconsiderably more study would be required to confirm this.

In order to test the possible fatigue of the cadmium borosilicateglasses of this invention, example 1 was cycled in the following manner:

1. Exposed to ultraviolet radiation (3650A.) produced by a commercialMineralite" long wave ultraviolet lamp having a 9-watt input, the outputbeing filtered to remove a major proportion of the visible energy, for15 minutes;

2. Exposed to infrared radiations at 500 watts for 15 minutes;

'3. Cooled at room temperature for 30 minutes.

The heating acted to bleach the color out of the glass more rapidly thanthe normal fading thereof when the actinic radiation is merely withdrawnfrom the glass. The optical transmittance of the glass decreased fromabout 92 to 43 percent after the first exposure to the ultravioletradiation and in the 1,250th cycle the glass decreased in transmittancefrom about 91 to 44 percent. Such findings indicate that these glassesare subject to virtually no fatigue.

In order to determine the fading rate of these glasses at roomtemperature, the actinic radiation was removed from the surface of theglass by means of a commercial cutoff filter opaque to radiationsshorter than 5000A. and the transmittance of the glass continuallyrecorded, the time being noted that it takes to recover to the geometricaverage (VT,,X- T of the initial transmittance (T and the transmittanceat infinite exposute (T This value is referred to as the halffading time(h,,).

Table [I clearly illustrates the photochromic behavior of the glasses ofthis invention. Each example was exposed to the commercial Mineraliteforminutes, this time arbitrarily being deemed to constitute infiniteexposure.

Table ll Example No. T oo I:

I 92 43 33 minutes 2 65 45 minutes 3 64 45 minutes 4 90 40 16 minutes 580 35 10 minutes 6 60 14 minutes I claim:

1. A photochromic article comprising a body of glass consistingessentially, in mole percent on the oxide basis, of about 45-70 percentCdO, 10-45 percent B 0 and 5-35 percent Si0 2. A glass composition whichis potentially photochromic consisting essentially, in mole percent onthe oxide basis, of about 45-70 percent CdO, 10-45 percent B 0 and 5-35percent SiO-,.

3. A glass composition according to claim 2 also containing up to 0.05percent by weight CuO and/or up to 0.5 percent by weight Ag.

2. A glass composition which is potentially photochromic consIstingessentially, in mole percent on the oxide basis, of about 45-70 percentCdO, 10-45 percent B2O3, and 5-35 percent SiO2.
 3. A glass compositionaccording to claim 2 also containing up to 0.05 percent by weight CuOand/or up to 0.5 percent by weight Ag.